In the past, various kinds of photodetectors such as photodiode, phototransistor, and CCD image sensor have been known, and widely used in the applications of photoelectric sensor for detecting the presence or absence of an object according to a change in received light amount, optical communication using light as the transmission medium, distance sensor for optically measuring distance by using the principal of triangulation, or the time difference or the phase difference between projected and received lights, and image pickup device for video and digital cameras.
In this kind of photodetector, when the received light amount is within an appropriate range, a received light output with a magnitude corresponding to the received light amount can be obtained. However, when the received light amount becomes excess, there is a problem that the received light output is saturated. For example, under the condition that there is environmental light such as sunlight, when using the photodetector for the photoelectric sensor for detecting intruders, the optical communication device such as optical remote control units, the distance sensor for autofocus cameras and robot's eyes, and the image pickup device used in cooperation with a light emitting source to obtain a distance image, the environmental light other than light irradiated from the light emitting source is incident on the photodetector, so that the received light amount increases, as compared with the case of receiving only the light irradiated from the light emitting source. However, there is a limitation with respect to amounts of carriers that the photodetector can generate. Therefore, when the received light amount excessively increases, the generation amounts of the carriers will be saturated. Consequently, a dynamic range of the photodetector reduces depending on the light amount of environmental light. This leads to an inconvenience that the received light output for signal light cannot be stably obtained. This problem is also known as a “blooming effect” in the field of the CCD image sensor.
To improve the above problem, for example, Japanese Patent Early Publication No. 62-272773 proposes that a large positive voltage is applied to an electrode formed on a p-type silicon substrate through an insulating layer to place a potential well at a reversed state, so that large amounts of carriers collected at an interface with the insulating layer are recombined with excessive amounts of carriers to control small amounts of carriers accumulated under the electrode to be not greater than predetermined amounts. However, according to this method, the excessive amounts of carriers generated by receiving light are recombined by using electric charges supplied from an external circuit for providing a reference potential of the substrate, i.e., large amounts of carriers (when using a p-type substrate, the carriers are holes, and when using an n-type substrate, the carriers are electrons) previously filled in the substrate. In addition, it is essential to change an operation frequency for recombining surface charges in accordance with exposure time. Therefore, there is a problem that the control becomes complex to obtain a received light output within an appropriate range by suppressing the dynamic range of the received light amount.
On the other hand, when a signal light and environmental light are mixed, variations in environmental light makes difficult to separate the signal light from the environmental light. As a technology of separating the signal light from the environmental light, it is proposed to use an optical filter for passing only the signal light having a specific wavelength. However, since the environmental light such as sunlight has a wide spectrum range, the influence of the environmental light cannot be sufficiently removed by only the optical filter.
To improve the above problem, for example, Japanese Patent Early Publication No. 2001-337166 proposes that a received light output obtained by a photodetector in an extinction period, which is a period where a signal light is not obtained, and light is not radiated from a light emitting source, is used as a component corresponding to only the environmental light, and a received light output obtained by the photodetector in a lighting period, which is a period where the signal light is obtained, and the light is radiated from the light emitting source, is used as a superimposed component of the signal light and the environmental light. In this case, a component corresponding to only the signal light can be extracted by subtracting the received light output of the photodetector in the extinction period from the received light output of the photodetector in the lighting period. However, a dynamic range for the signal light of the photodetector decreases in the presence of the environmental light. When the photodetector is saturated, there is still a problem that a large received light output cannot be extracted with respect to the signal light. In brief, in the presence of the environmental light, even when the light amount emitted from the light emitting source is increased, or the light receiving time of the photodetector is extended, it is difficult to obtain a sufficiently large S/N ratio due to a reduction in dynamic range for the signal light of the photodetector.